1. Field of the Invention
The present invention relates generally to methods and devices for the nondestructive evaluation of materials. The present invention relates more specifically to the use of magnetostrictive sensors to inspect railroad rails to detect broken rails and cracked rails remotely, namely, from an operating train projecting signals forward of the train and broken or cracked rails reflecting signals back to magnetostrictive sensors on the train in time for the train to stop.
2. Description of the Related Art
Magnetostrictive effect refers to the phenomena of a physical dimension change in ferromagnetic materials that occurs through variations in magnetization. In magnetostrictive applications, the generation and detection of mechanical waves is typically achieved by introducing a pulse current into a transmitting coil adjacent to a ferromagnetic material. The change in magnetization within the material located near the transmitting coil causes the material to change its length locally in a direction parallel to the applied field. This abrupt local dimension change, which is the magnetostrictive effect, generates a mechanical wave that travels at the speed of sound within the ferromagnetic material. When the mechanical wave is reflected back from the end of the ferromagnetic material, or from a defect in the ferromagnetic material, and reaches a detection coil, the mechanical wave generates a changing magnetic flux in the detection coil as a result of the inverse magnetostrictive effect. This changing magnetic flux induces an electric voltage within the detection coil that is proportional to the magnitude of the mechanical wave. The transmitting coil and the detection coil can be identical.
Advantages of using the magnetostrictive effect in nondestructive evaluation (NDE) applications include (a) the sensitivity of the magnetostrictive sensors, (b) durability of the magnetostrictive sensors, (c) no need to couple the sensor to the material being investigated, (d) long range of the mechanical waves in the material under investigation, (e) ease of implementation, and (f) low cost of implementation.
The use of magnetostrictive sensors (MsS) in the nondestructive evaluation (NDE) of materials has proven to be very effective in characterizing defects, inclusions, and corrosion within various types of ferromagnetic and non-ferromagnetic structures. A MsS launches a short duration (or a pulse) of elastic guided waves in the structure under investigation and detects guided wave signals reflected from anomalies such as defects in the structure. Since guided waves can propagate long distances (typically 100 feet or more), the MsS technique can inspect a global area of a structure very quickly. In comparison, other conventional NDE techniques such as ultrasonics and eddy currents inspect only the local area immediately adjacent to the probes used. Therefore, the use of magnetostrictive sensors offers a very cost effective means for inspecting large areas of steel structures such as strands, cables, pipes, and tubes quickly with minimum support requirements such as surface preparation, scaffolding, and insulation removal. The ability to use magnetostrictive sensors with little preparation of the object under inspection derives from the fact that direct physical contact between the sensors and the material is not required.
Efforts have been made in the past to utilize magnetostrictive sensor technologies in association with the inspection of both ferromagnetic and non-ferromagnetic materials. Included in these efforts are systems described in U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, which are each commonly owned by the assignee of the present invention. The disclosures of U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, provide background on the magnetostrictive effect and its use in NDE and are therefore incorporated herein by reference. These efforts in the past have focused primarily on the inspection of pipe, tubing, and steel strands/cables wherein the geometry of the structure is such that the cross-sectional diameter is small in comparison to the length of the structure. While these systems and their application to longitudinal structures find significant applications, there are yet other structures that could benefit from the use of magnetostrictive based NDE.
Other efforts have been made in the past to utilize sensors that measure magnetic flux and/or acoustic waves in structural materials. These efforts have included those described in the following patents:
U.S. Pat. No. 3,555,887 issued to Wood on Jan. 19, 1971 entitled Apparatus for Electroacoustically Inspecting Tubular Members for Anomalies Using the Magnetostrictive Effect and for Measuring Wall Thickness. This patent describes a system designed to direct a mechanical wave through the thickness dimension of a long tubular member. The sensitivity of the device is limited to the directing of a wavefront normal to the surface of the material under inspection and immediately back to a sensor when reflected from an opposite wall or an anomaly.
U.S. Pat. No. 4,881,031 issued to Pfisterer, et al. on Nov. 14, 1989 entitled Eddy Current Method and Apparatus for Determining Structure Defects in a Metal Object Without Removing Surface Films or Coatings. This patent describes a method for establishing localized eddy currents within ferromagnetic materials and recognizes the presence and effect of a coating in order to identify and quantify corrosion beneath the coating. As with other eddy current methods, the ability to inspect a material is limited to the area immediately adjacent to the sensor.
U.S. Pat. No. 5,544,207 issued to Ara, et al. on Aug. 6, 1996 entitled Apparatus for Measuring the Thickness of the Overlay Clad in a Pressure Vessel of a Nuclear Reactor. This patent describes a system directed solely to the measurement of magnetic field variations that result from the distribution of the magnetic field through overlays of varying thickness. The system utilizes a magnetic yoke that is placed in close contact with the surface of the overlay clad of the pressure vessel.
U.S. Pat. No. 5,687,204 issued to Ara, et al. on Nov. 11, 1997 entitled Method of and Apparatus for Checking the Degradation of a Pressure Vessel of a Nuclear Reactor. This patent describes a system similar to the earlier issued Ara, et al. patent and utilizes a magnetic yoke having an excitation coil and a magnetic flux measuring coil that are placed in close contact with the inner wall of the pressure vessel. The hysteresis magnetization characteristics formed by the magnetic yoke and the pressure vessel wall are measured. Degradation of the material comprising the pressure vessel is inferred from a determination of the hardness of the material which is determined from the coercive forces obtained by analyzing the hysteresis characteristics of the magnetization.
The nondestructive evaluation of materials using magnetostrictive sensors is based upon the magnetostrictive effect and its inverse effect, and the phenomenon that causes the physical dimensions of a ferromagnetic material to change slightly when the material is magnetized or demagnetized or otherwise experiences a changing magnetic field. The inverse effect is a phenomenon that causes a magnetic flux in the material to change when the material is stressed. Systems utilizing magnetostrictive sensors use the magnetostrictive effect and its inverse effect to generate and detect guided waves that travel through the ferromagnetic material.
In general, a magnetostrictive sensor consists of a conductive coil and a means for providing a DC bias magnetic field in the structure under inspection. The means for providing a bias magnetic field can include the use of either permanent magnets or electromagnets. In a transmitting magnetostrictive sensor, an AC electric current pulse is applied to the coil. The resulting AC magnetic field (a changing magnetic field) produces elastic waves (also known as guided waves) in an adjacent ferromagnetic material through the magnetostrictive effect. For pipes, cables, tubes, and the like, the waves are launched along the length of the longitudinal structure. In the receiving magnetostrictive sensor, a responsive electric voltage signal is produced in the conductive coil when the elastic waves (transmitted or reflected from anomalies within the material) pass the sensor location, through the inverse magnetostrictive effect.
With MsS techniques, defects are typically detected by using the pulse-echo method well known in the field of ultrasonics. Since the sensor relies on the magnetostrictive behavior found in ferromagnetic materials, this technology is primarily applicable to the inspection of ferromagnetic components such as carbon steel piping or steel strands. It is also applicable, however, to the inspection of nonferrous components if a thin layer of ferromagnetic material, such as nickel, is plated or coupled onto the component in the area adjacent to the magnetostrictive sensors.
The magnetostrictive sensor technique has the advantage of being able to inspect a large area of material from a single sensor location. Such sensors have, for example, been used to accurately inspect a length of pipe or cable of significantly more than 100 feet. Further, magnetostrictive sensor techniques are comprehensive in their inspection in that the methods can detect both internal and external defects, thereby providing a 100% volumetric inspection. The techniques are also quite sensitive, being capable of detecting a defect with a cross-section less than 1% of the total metallic cross-section of cylindrical structures such as pipes, tubes, or rods. Finally, as indicated above, magnetostrictive sensor techniques do not require direct physical contact between the component surface and the sensor itself. This eliminates the need for surface preparation or the use of a couplant.
Application to Railroad Rails
Defects can be found in railroad rails as well. Various types of rail failure modes exist. One failure mode is the shell defect. The shell defect tends to propagate down the length of the rail. The shell defect is somewhat benign until it turns into a transverse defect, at which time it can cause rail failure. It is desirable to use the magnetostrictive sensor technique for detecting and locating various anomalies within railroad rails and more particularly transverse defects that exist under the shells in the rail. This need results from the inability of the acoustic wave of conventional ultrasonic inspection to penetrate below the shell.
Under these conditions, a full volumetric inspection approach that has the ability to propagate acoustic energy below the shell is required such as that provided by the inspection technique of the present invention. The present technique is ideal because it is a nonintrusive inspection technique. In addition, a manner of inspection is desired that allows the magnetostrictive sensor of the present technique to be mounted on a moving inspection vehicle such as a train. The reflected signal from the defect can be of sufficient length to allow the train to stop prior to reaching the defect or, in the alternative, the defect location is recorded for subsequent repair.
It is therefore an object of the present invention to provide a method for implementing magnetostrictive based NDE of rails and to determine the presence of anomalies indicative of defective rails.
It is a further object of the present invention to provide a method for using magnetostrictive sensors for the inspection of rails that is capable of transmitting and receiving guided waves within the rail and generating signals representative of the characteristics of such waves appropriate for the analysis and detection of anomalies in the rail.
It is a further object of the present invention to provide a method for the inspection of rails that includes the use of a magnetostrictive sensor specifically adapted for directing guided waves into the rails and detecting such waves as may be reflected from anomalies within the structure.
It is a further object of the present invention to provide a method and apparatus for the nondestructive evaluation of rails utilizing magnetostrictive sensors that are capable of investigating large lengths of rail from long distances.
It is yet another object of the present invention to provide a method and apparatus for nondestructive evaluation of rails having ferromagnetic materials through the use of a magnetostrictive sensor that may operate either in the symmetrical or anti-symmetrical Lamb wave mode.
It is yet another object of the present invention to provide a method and apparatus for nondestructive evaluation of rails utilizing magnetostrictive sensors that generate and detect shear horizontal waves in the item being inspected.
It is yet another object of the present invention to provide a method and apparatus for inspecting rails utilizing magnetostrictive sensors.
Yet another object of the present invention is to provide a method and apparatus for inspecting rails using magnetostrictive sensors that propagate guided waves along the length of the rail.
Still another object of the present invention is to provide a method for using magnetostrictive sensors for the inspection of rails that is capable of transmitting and receiving guided waves within the rail without making physical contact with the rail.
Yet another object of the present invention is to provide a method and apparatus for the nondestructive evaluation of rails utilizing magnetostrictive sensors that can be attached directly in front of a train as it transmits and receives guided waves within the rail.
It is another object of the present invention to provide a method and apparatus for the nondestructive evaluation of rails utilizing magnetostrictive sensors that generate a low frequency elastic wave that travels distances up to several miles in a continuous rail.
It is yet another object of the present invention to provide a method and apparatus for the nondestructive evaluation of rails utilizing magnetostrictive sensors that transmit and receive elastic waves sensitive to broken rail at distances of approximately one mile from the magnetostrictive sensor.
Still another object of the present invention is to provide a method and apparatus for the nondestructive evaluation of rails utilizing magnetostrictive sensors that work in conjunction with a data acquisition and analysis system that can acquire and analyze the data using computer software.
Yet another object of the present invention is to provide a method and apparatus for the nondestructive evaluation of rails utilizing magnetostrictive sensors that works in conjunction with a data acquisition and analysis system which output can be used to initiate a warning to the train engineer or generate control signals that can be used to automatically brake the train.
In fulfillment of these and other objectives, the present invention provides a method and apparatus for implementing magnetostrictive sensor techniques for the nondestructive evaluation of plate type structures such as rails. The system includes magnetostrictive sensors specifically designed for application in conjunction with plate type structures that generate guided waves in the plates which travel through the plate in a direction parallel to the surface of the plate. Similarly structured sensors are positioned to detect the guided waves (both incident and reflected) and generate signals representative of the characteristics of the guided waves detected. The system anticipates the use of either discrete magnetostrictive transmitters and receivers or the use of a single magnetostrictive sensor that operates to both transmit and detect the guided waves. The sensor structure is longitudinal in nature and generates a guided wave having a wavefront parallel to the longitudinal direction of the sensor. Appropriate electronics associated with the process of generating the guided waves and controlling the propagation direction of the generated wave through the magnetostrictive transmitter as well as detecting, filtering, and amplifying the guided waves at the magnetostrictive receiver, are implemented as is well known in the art. Signal analysis techniques, also known in the art, are utilized to identify anomalies within the plate type structure and welds therein. The method utilizes pattern recognition techniques as well as comparisons between signal signatures gathered over time from the installation of the structure under-investigation to a later point after deterioration and degradation may have occurred.
By rotation of the magnetic field by 90xc2x0, the magnetostrictive sensor can be changed from operating in the symmetrical or the anti-symmetrical Lamb wave mode to a horizontal shear wave that is applied to the ferromagnetic material being inspected. In the horizontal shear wave mode, the DC bias magnetic field is in a direction perpendicular to the direction of wave propagation.
When magnetostrictive sensors are used to inspect rails, the guided wave travels in both directions in the rail at a periodic repetition rate, and any cross-sectional defect in the rail will reflect or scatter the propagating elastic wave. The receiver core/coil then detects the reflected wave using the reciprocal wave generation process. The defect sensitivity is a function of the size of the defect and the frequency of the elastic wave.